Immersion type electroless gold plating liquids have been used to form an intermediate layer in an effort to improve the solder adhesion of circuits, terminals, and so forth in printed wiring boards, and to improve the adhesion of reductive gold plating and the like. Most of the gold plating liquids used for this purpose contain a toxic cyanide compound as a gold compound, but concerns for environment and workplace require cyanide-free gold plating liquids that do not contain toxic substances.
Patent applications that have been filed for cyanide-free substitutional gold plating liquids include those that make use of gold sulfite compounds (see, for example, Japanese Patent No. 3,030,113 and Japanese Patent Publication No. 2003-13249), those that make use of a salt of gold sulfites or chloroaurates (see, for example, Japanese Patent Publication No. H8-291389), and those that make use of gold sulfite, gold chloride, gold thiosulfate, or gold mercaptocarboxylates (see, for example, Japanese Patent Publication No. H10-317157). The electroless gold plating liquids discussed in these are cyanide-free and therefore low in toxicity, and can be used close to neutral. But, a problem is their inferior solder adhesion and plating film adhesion. “Plating film adhesion” refers to the adhesion between an immersion type electroless gold plating film and the substrate and, when an immersion type electroless gold plating film is used as an intermediate layer, refers to the adhesion between the layers above and below the film.